The wide variety of applications of metal nanoparticles has motivated many studies of their properties. Some important practical issues are how the size, composition and structure of these materials affect their catalytic and optical properties. In this article we review our recent work on the photophysics of metal nanoparticles. The systems that have been investigated include Au particles with sizes ranging from 2 nm diameter (several hundred atoms) to 120 nm diameter, and bimetallic core-shell particles composed of Au, Ag, Pt and/or Pb. These particles, which have a rather narrow size distribution, are prepared by radiolytic techniques. By performing time-resolved laser measurements we have been able to investigate the coupling between the electrons and phonons in the particles, and their low frequency "breathing" modes. These experiments show that for Au the time scale for electron-phonon coupling does not depend on size, in contrast to metals such as Ga and Ag. On the other hand, the frequency of the acoustic breathing modes strongly depends on the size of the particles, as well as their composition. These modes are impulsively excited by the rapid lattice heating that accompanies ultrafast laser excitation. The subsequent coherent nuclear motion modulates the transmitted probe laser intensity, giving a "beat" signal in our experiments. Unlike quantum-beats in molecules or semiconductors, this signal can be completely understood by classical mechanics.

The low-frequency acoustic breathing modes in Au-Pb core-shell nanoparticles have been studied by timeresolved spectroscopy. The frequency of the breathing mode was determined for a series of samples with a 47 nm diameter Au core and Pb shells of different thickness. The measured frequencies decrease with increasing Pb content, but not to the extent expected from a classical model for a homogeneous sphere. These results show that the acoustic breathing modes of metallic core-shell particles are significantly perturbed when the two metals have different elastic properties.

A silver colloid (mean particle size 6 nm) is prepared via photochemical reduction of Ag+ ions. After total silver ion reduction, the colloid exhibits an extraordinarily intense surface plasmon absorption band. The absorption band is strongly damped by small concentrations of carbon disulfide, thiophenol, and diethyl disulfide. Carbon disulfide is irreversibly adsorbed, and so is diethyl disulfide. The irreversible adsorption is attributed to the high reactivity of the nanoparticles due to their low Fermi level potential. It is proposed that this in turn leads to dissociative adsorption of the disulfides onto the silver particles. Weaker damping is observed for thiophene, thiourea, and diethyl sulfide. The damping of the plasmon absorption band is attributed to the electronic distortion of a thin layer within the particle by the adsorbate−particle interaction.

The reactions of organic radicals, which are generated by pulse radiolysis in aqueous K 2 PtCl 4 solutions containing an alcohol or ether, are traced by optical absorption and conductivity methods. Upon attack by 1-hydroxyalkyl or 1-alkoxyalkyl radicals, PtCl 4 2-disappears in a chain reaction, in which Cl -ligands are substituted by H 2 O. 2-Hydroxyalkyl radicals do not initiate the chain. The final product of attack by 1-hydroxyalkyl radicals is colloidal platinum.